Liquid cooling is becoming widely understood in the field of electronics cooling. Rather than use air to cool heat producing components, liquid is used to absorb and transport the heat. Liquids are typically much more efficient than air as a heat transfer fluid since they have considerably higher thermal conductivities, specific heat capacities and provide the opportunity for phase change processes.
Liquid cooling systems used to cool commercial type electronics generally utilize a closed loop process. Although some liquid cooling systems, such as heat pipes, are passive, active systems provide system flexibility needed for most cooling applications. With active systems, the fluid is pressurized via one or more pumps. The pressurized fluid is delivered to the cooling module wherein the fluid absorbs heat from an electronic component. The fluid leaving the cooling module is transported to a heat exchanger wherein the heat is removed from the fluid. Many other components, such as filters, may be placed into the closed loop system for additional system functionality.
There are many ways for the liquid of a cooling system to absorb heat from the heat producing device. Forced convection is often used by cold plates. Cold plates replace heat sinks and keep the fluid separate from the electronics to be cooled. Inside the cold plate can be features such as micro-channels and mini-channels, which increase the surface area and overall heat transfer. Cold plates are required for use with conductive liquid cooling fluids, such as water. Water must be kept separate from the electronics since it would obviously short out the circuits and cause irreparable harm to the electronic systems and components.
Dielectric fluids are electrically inert to electronic components. Depending upon the exact type of fluid, dielectric fluids typically have breakdown voltages in excess of air. Fluorinert (a trademark of 3M) is a commercially available dielectric fluid that has been used in liquid cooling systems for several decades. Most electronic components can be placed directly in contact with dielectric fluids, such as Fluorinert. By placing the fluid in direct contact with components to be cooled, heat transfer rates can be much greater than typical cold plate systems. In addition, entire boards having many electronic components of varying height and shapes can be cooled by a single liquid cooling system. In addition to Fluorinert, other dielectric fluids are available such as commonly used refrigerants.
A significant challenge in using dielectric fluids within closed loop cooling systems is that they typically have poor heat transfer properties. Fluorinert, for example, has a thermal conductivity value in the range of air. Water has a thermal conductivity several orders of magnitude greater. The result is that it can be difficult to get heat into dielectric fluids. To make dielectric fluids effective, two-phase cooling can be employed. Rather than use single phase heat transfer wherein a fluid is heated up by sensible heat gains, two-phase cooling takes advantage of changing liquid into vapor. Liquid absorbs heat and transforms into vapor which requires substantial amounts of energy. The vapor is condensed by a heat exchanger back to liquid form. The result is a highly efficient system. Preferably, thin-film evaporative cooling is used to maximize heat transfer coefficients.
Spray cooling is an ideal thin-film cooling method for producing effective and efficient dielectric liquid cooling systems. Fluid is pumped via a pump and delivered to a spray module. Nozzles, preferably atomizers, break up the liquid into small drops that travel to the cooling surface with significant momentum. The drops create a very thin coolant film which readily changes phase to a vapor state. The drops also entrain vapor within the module. Entrained vapor can be used to further reduce the thickness of the cooling film and reduce localized pressures, both results increase heat transfer coefficients. Spray cooling can be used within cold plates for systems that have localized heat sources, and can also be used within global type systems that spray the fluid directly on entire electronic systems. A coldplate spray cooling system is described by U.S. Pat. No. 7,104,078 and U.S. Pat. No. 5,220,804; and a global spray cooling system is described by U.S. Pat. No. 5,880,931 and U.S. Pat. No. 6,976,528.
Global spray cooling allows electronics to be used in revolutionary ways. By creating a completely sealed chamber for the electronics, the systems can be used in environments that may be nearly void of air, under water, and in extreme hot and cold. Global cooling is also ideal for benign environments, such as data centers. Global cooling can be used for blade servers which provides the ability to create very power dense systems and racks.
One problem with prior art global cooling systems is that because the electronics reside within a single chamber, the entire computing system must be shut down in order to provide service and maintenance of a single board within. Although the maintenance and service requirements may be perfectly acceptable for global cooling systems used in airplanes and standalone systems, the requirements are not optimal for use with systems that must be used in 7×24 applications. Computers used in data centers for example, are expected to be plug and play.
One method of creating hot-swappable electronics within a global cooling system is the “clamshell” method, such as described by U.S. Pat. No. 6,955,062. Cards are completely enclosed by a chamber. Each chamber has an inlet and an outlet comprised of quick disconnect couplings or blind mate connectors. The quick disconnect couplings are part of an electrical backplane. Each chamber can be removed from the backplane assembly individually. A weakness in the clamshell design is that the backplane assembly is not environmentally isolated. The result is that the system can not be placed in very harsh working environments. In addition, the cards typically have to be custom designed in order to interface with the clamshell housing.
Another problem with prior art global cooling systems that utilize single and large chambers for cooling, is that a significant amount of the fluid within is open to the environment during service and maintenance. Perfluorocarbon type fluids are very stable molecules, and hence, there is very little risk in the fluid breaking down. Perfluorocarbon type fluids do have the ability to absorb significant amounts of air. During maintenance cycles, it is disadvantageous to have the cooling fluid absorb air, or non-condensable gases, as during operation the non-condensable gases cause increased system pressures, saturation temperatures and temperatures of electronics. Other fluids, such as the Novec family (a trademark of 3M), may break down into other substances with the introduction of water, or water vapor. Prior art single chamber global cooling systems do provide the means to reduce the exposure of the fluid within the system to fluid and gases as part of the environment.
In these respects, the multi-chamber cooling system according to the present invention substantially departs from conventional concepts of the prior art, and in doing so provides an apparatus primarily designed to provide global cooling to electronics while allowing cards to be hot-swapped.